Question

if p°A & P°B are 108 and 36 torr respectively.What will be the mole fraction of A is the vapour phase if B has mole fraction in solution 0.5

Answer 1

Hello Dear,

◆ Answer -

Mole fraction of A in vapour phase = 0.75

● Explanation -

In the solution, total pressure will be -

P = XA.p°A + XB.p°B

P = 0.5 × 108 + 0.5 × 36

P = 72 torr

Mole fraction of A in vapour phase -

XA' = XA.p°A / P

XA' = 0.5 × 108 / 72

XA' = 0.75

Therefore, mole fraction of A in vapour phase is 0.75 .

Hope this helped you.

Answer 2

Applying Roult's law,

Total Vapour Pressure of the solution = Pa + Pb

= P°a × Xa + P°b × Xb

= 108 × (1 - Xb) + 36 × Xb

= 108 × (1 - 0.5) + 36 × 0.5

= 108 × 0.5 + 36 × 0.5

= 54 + 18

= 72 torr.

Now, Mole Fraction in Vapour Pressure,

  Applying Dalton's law,

 Partial vapor pressure = Total Vapour Pressre × Xa

    108 × 0.5 = 72 × Xa

     Xa = 54/72

  ∴ Xa = 0.75

Hope it helps.